Plasma arc torches, also known as electric arc torches, are commonly used for cutting, marking, gouging, and welding metal workpieces by directing a high energy plasma stream consisting of ionized gas particles toward the workpiece. In a typical plasma arc torch, the gas to be ionized is supplied to a distal end of the torch and flows past an electrode before exiting through an orifice in the tip, or nozzle, of the plasma arc torch. The electrode has a relatively negative potential and operates as a cathode. Conversely, the torch tip constitutes a relatively positive potential and operates as an anode. Further, the electrode is in a spaced relationship with the tip, thereby creating a gap, at the distal end of the torch. In operation, a pilot arc is created in the gap between the electrode and the tip, often referred to as the plasma arc chamber, which heats and subsequently ionizes the gas. The ionized gas is blown out of the torch and appears as a plasma stream that extends distally off the tip. As the distal end of the torch is moved to a position close to the workpiece, the arc jumps or transfers from the torch tip to the workpiece with the aid of a switching circuit activated by the power supply. Accordingly, the workpiece serves as the anode, and the plasma arc torch is operated in a “transferred arc” mode.
During operation of the plasma arc torch, both the electrode and the tip, among other components, are subjected to extremely high temperatures and severe conditions from the high current, gas flow, and the plasma stream, in addition to chemical reactions with different types of gases at high temperatures. These conditions are especially intense within the plasma arc chamber, and as a result, cause wear of the electrode and the tip over time. With increased wear, the performance of these components degrades, causing the plasma stream to become less controlled and constricted, which eventually effects cut quality of the plasma arc torch in a negative manner. Thus to maintain an acceptable cut quality, the components such as the electrode and the tip must be periodically replaced, hence the reference to these components as “consumable components.”
In most electrodes for plasma arc torches, an emissive insert is disposed within a distal end of the electrode face. The emissive insert is typically a material such as Hafnium, and thus provides a location for arc attachment and transfer during operation due to its inherent ability to transfer electrons more efficiently than other materials. During operation however, the Hafnium wears according to several mechanisms depending on the stage of the cutting process. During plasma arc ignition, the primary wear mechanism is related to high ion flux pressures and electromagnetic pressures, as well as possible cracking and loss of an oxide layer on the surface, whereas during cutting, the primary wear mechanism is evaporation. At plasma arc shut off, a surge of gas within the plasma arc chamber has a tendency to displace the Hafnium, which becomes molten with the extremely high temperatures. Accordingly, the displacement of molten Hafnium increases wear of the electrode and decreases the usable life of the electrode. Additionally, Hafnium is a relatively expensive material and thus it is desirable to reduce wear and replacement of the electrodes as much as possible.
Therefore, a need exists in the art to provide improved techniques for extending the life of consumable components for use in plasma arc torches.